Recently, particulates contained in exhaust gases that are discharged from internal combustion engines of vehicles, such as cars, buses and trucks, and construction machines, have raised serious problems as those particulates are harmful to the environment and the human body. Conventionally, various honeycomb structural bodies (honeycomb filters), which allow exhaust gases to pass through porous ceramics to collect particulates in the exhaust gases so that the exhaust gases are purified, have been proposed.
FIG. 1 is a perspective view that schematically shows one example of a honeycomb structural body. FIG. 2(a) is a perspective view that schematically shows a pillar-shaped porous honeycomb small-size member used for forming the honeycomb structural body shown in FIG. 1, and FIG. 2(b) is a cross-sectional view taken along line A-A of FIG. 2(a).
As shown in FIG. 1, the honeycomb structural body 10 has a structure in which a plurality of pillar-shaped porous honeycomb small-size members 30, made from a ceramic material such as silicon carbide and the like, are combined with one another through adhesive layers 14 to form a pillar-shaped porous honeycomb member 15 with a sealing material layer 13 being formed on the circumference of the pillar-shaped porous honeycomb member 15.
As shown in FIG. 2, the pillar-shaped porous honeycomb small-size member 30 has a structure in which a number of through holes 31 are placed side by side in the length direction and partition wall 33, which separates the through holes 31 from each other, are allowed to serve as a filter. In other words, as shown in FIG. 2(b), each of the through holes 31 that are formed in the pillar-shaped porous honeycomb small-size member 30 is sealed with a plug 32 at either one of ends on its exhaust gas inlet side or exhaust gas outlet side so that exhaust gases that have entered one through hole 31 are discharged from another through hole 31 after having always passed through a partition wall 33 that separates the through holes 31. Here, arrows in FIG. 2(b) indicate flows of exhaust gases.
When an exhaust gas purifying device, provided with the honeycomb structural body 10 having such a structure, is placed in an exhaust passage in an internal combustion engine, particulates in exhaust gases discharged from the internal combustion engine are captured by the partition wall 33 when passing through the honeycomb structural body 10 so that the exhaust gases are purified.
The sealing material layer 13 is placed so as to adjust the shape of a honeycomb structural body 10, prevent exhaust gases from leaking through the peripheral portion of the honeycomb structural body 10 and improve the heat-insulating property of the honeycomb structural body 10, when the honeycomb structural body 10 is placed in an exhaust passage in an internal combustion engine.
FIG. 5 is an explanatory view that schematically shows one example of a conventional process in which a sealing material paste layer is formed on the periphery of a pillar-shaped porous honeycomb member. Moreover, FIG. 6 is an explanatory view that schematically shows a state of a sealing material layer in the conventional honeycomb structural body.
Conventionally, in order to form a sealing material layer 52 on the periphery of a pillar-shaped porous honeycomb member 50, first, the pillar-shaped porous honeycomb member 50 is supported at the axis in the length direction, and while the pillar-shaped porous honeycomb member 50 is being rotated around the axis in the length direction, paste-like sealing material, which forms a material for the sealing material layer 52, is adhered to the peripheral face of the pillar-shaped porous honeycomb member 50.
Next, as shown in FIGS. 5(a) to 5(e), a plate-shaped scraper 51, which is arranged so as to have a fixed angle and an interval with respect to the circumferential face of the rotating pillar-shaped porous honeycomb member 50, is used for forming the sealing material paste layer, and in this case, the scraper 51 is pressed onto the paste-like sealing material so that the paste-like sealing material is expanded with an almost uniform thickness over the entire circumferential face to form a sealing material paste layer. Lastly, the sealing material paste layer, thus formed, is dried and solidified (see Patent Documents 1 and 2).
However, in the case when the sealing material layer 52 is formed on the periphery of the pillar-shaped porous honeycomb member 50 in this method, the external dimension of the honeycomb structural body 54 thus manufactured tends to fail to strictly coincide with the designed dimension, and, for example, in the case of a pillar-shaped honeycomb structural body 54, the profile of a cross section perpendicular to the length direction tends to disperse depending on respective cross sections. One of the reasons for this problem is that, when, after almost forming a sealing material paste layer on the circumferential face of a pillar-shaped porous honeycomb member 50 by using a plate-shaped scraper 51, the plate-shaped scraper 51 is separated from the circumferential face, a protruding separation trace 53 is undesirably formed on one portion of the sealing material layer 52, as shown in FIG. 6, due to high viscosity of the paste-like sealing material. Another reason for this problem is that, when a honeycomb structural body 54 having a shape other than a cylindrical shape is manufactured, the plate-shaped scraper 51 needs to be moved following the rotation of the pillar-shaped porous honeycomb member 50 so as to maintain a fixed angle and interval with respect to the peripheral face of the pillar-shaped honeycomb member 50; thus, the pressing force of the plate-shaped scraper 51 onto the circumferential face becomes uneven to cause irregularities in the thickness of the sealing material layer 52.
It has been found that the dispersion in the external dimension of the honeycomb structural body is easily generated when there are parallel grooves (irregularities) in the length direction on the peripheral portion of the pillar-shaped porous honeycomb member on which the sealing material past layer is formed. Moreover, regarding a honeycomb structure formed by combining a plurality of pillar-shaped porous honeycomb small-size members with one another, since it is difficult to form a pillar-shaped porous honeycomb member having the same shape over the entire circumferential face due to: unevenness in the thickness of the sealing material located between the pillar-shaped porous honeycomb small-size members; and warps and the like of the pillar-shaped porous honeycomb small-size members, the dispersion in the external dimension of the honeycomb structural body tends to occur more frequently.
In the case when the honeycomb structural body has dispersion in the profile as described above, the surface pressure of the honeycomb structural body becomes uneven when it is placed in an exhaust passage in an internal combustion engine through a holding sealing member made from heat-insulating fibers and the like, to cause a failure in firmly securing the honeycomb structural body in the exhaust passage and the subsequent deviation of the honeycomb structural body when exhaust gases are allowed to pass through it.
Here, in the honeycomb structural body, the sealing material layer normally is a portion that is less likely to pass exhaust gases through, in comparison with the pillar-shaped porous honeycomb member; therefore, when the honeycomb structural body is used as an exhaust gas purifying device, it has been necessary to make the thickness of the sealing material layer thinner in order to reduce the pressure loss.
Moreover, when the honeycomb structural member is used as a filter for an exhaust gas purifying device and the like, it is put into a metal case through a heat-insulating member made from ceramic fibers and the like, and heated to high temperatures. At this time, since the honeycomb structural body tends to cause a temperature difference in the length direction, it results in a problem that cracks occur in the sealing material layer due to the difference in coefficients of thermal expansion in the honeycomb structural body.
Patent Document 1
    JP-A 2000-5671Patent Document 2    JP-A 2000-102709